Some time in the spring a fashion editor told us that it would be the easiest summer ever: just hop into one of the season's ubiquitous mini-smock dresses, admire your long, tanned legs in silvery sandals, and you'd be good to go. Standing on the station concourse at Waterloo on a dank, overcast, July morning, a friend visiting from abroad watched the trains arrive and depart and the stream of rush-hour women descending to the tube, and she concluded that what British women were wearing this year was whatever they had in their wardrobes from last year.

Like unripened tomatoes rotting on the vine, the minidresses hung, neglected and unwanted. It was partly the fault of the weather (one afternoon in August I broke down and turned on the heating, after a couple of hours of teeth-chattering chill), partly the overoptimism of designers who believed that women can always be shamed into wearing what they don't like and what doesn't suit them by fear of frumpery. Or perhaps they just don't care about those women whose imperfect figures cause them to take into account "fit and flatter" when they shop. Let them wear Primark or George by Asda. In town centres, teenage girls with legs like stumps wore mini-smocks over leggings, but then a couple of years earlier they had bared their wobbly midriffs, for if you can't make a sartorial fool of yourself at 16, you might as well move to the French provinces. The rest of us became fashion refuseniks.

After wandering from shop to shop and finding nothing to buy, I decided to take the revolutionary step of actually buying nothing, to wait until autumn and invest in what, I was told by my Vogue spies, would be an entirely new look, one that would be worth the wait. On the whole, women can be classified as one of three shapes: the upright triangle, the upside-down triangle, and the Latvian supermodel. I'm prepared to believe, at a pinch, that the latter group possibly have their own clothes-buying difficulties ("Oh, no, I look so tall and slim in this!"), but I've only got 1800 words and you Latvian supermodels must find your consolation elsewhere. The majority of British women are upright triangles, otherwise known as pears. The mini-smock exposes our worst attribute, our legs, and hides the only attribute we don't want to cover up, our waists. The upside-down triangle, or apple, has no waist but good legs; on the other hand, most women over the age of 40 do not want to expose theirs to mid-thigh. You can argue about this, but the fact is they don't.

So what is there to buy this season? In my view, it's a good time for us pears. Here's the essential shape: the jacket with the nipped-in waist, marking the return of tailoring. I like tailoring. It can make you look like one of the more tragic characters in The Office if you stick to grey and boring accessories, but tailoring hides the lumpy bits that stick out, especially if you pay for quality. A cheap Zara jacket unfortunately goes on looking like a cheap Zara jacket. It never quite fits. Reluctantly, having been a big Zara fan, despite the hours sewing on dropped buttons and drooping hems, I've decided that you really do only get what you pay for. If I were buying a new jacket this season, I'd go to Jaeger, Hugo Boss, one of the top-tier high-street chains such as Reiss, Whistles, Hobbs and Jigsaw. I might also try - sit tight - Armani, who really does know how to fit and flatter, and the higher you move up the hierarchy of his labels, the better the fit. Jackets have been out for so long that, now they're back again, they're bound to be around for a while, and a jacket that will last more than one winter is what is called investment dressing. And you can always sell it on eBay, where proper labels can often recoup at least 50% of what you originally paid for them.

Below the jacket there are a couple of options. Wide-legged trousers are one. I concede that if you are under 5ft 3in, this probably isn't going to be the best look for you, making you look much like a galleon in full sail, flipped over, but I am 5ft 5in and can testify to their fantastic leg-lengthening properties. "Wide-leg trousers make me look like a box," I was told, doubtfully, by a friend. The wrong kind of wide-leg trousers can indeed make you look like a rectangular block of wood, but you need to get the type that flare in an A-line from the top of the thighs. The other option is the calf-length skirt, of which my esteemed, 20-something mini-shift-wearing colleague Hadley Freeman has written dismissively in these very pages. It's true that calf-length skirts do indeed cut you off at the middle of the calf and make your legs look shorter, but that's only if you're wearing them with shoes. You have to wear them with boots.

So picture, if you will, a nice little jacket with a waist, which you can buy pretty much anywhere at the moment; a button-down calf-length skirt and, say, suede boots. Or wide-legged trousers (they need to be really wide and have a kind of flow to them, rather than descending stiffly to the ground) worn with suede high-heeled ankle-boots (I would send you to Marks and Spencer for a pair in the Autograph range that are just coming in now). Grown-up, sexy. Suitable for work, and suitable for parties.

And to go with all that, a really killer handbag - by which I don't mean an overpriced It bag with its designer label glued on in China, or a fake made by child labourers in Thailand who have had their legs broken so they won't go out to play in their lunch minute (true story), but rather a judicious purchase made by a niche designer such as Anya Hindmarch. Because, as my mother always said - and indeed, we put it in her death notice in the Jewish Chronicle - a good handbag makes the outfit.

Black, grey, patent. These are the key trends for autumn and you don't get much less teenage than that, though there is always the problem of rendering yourself invisible, of using clothes as concealment: I'm middle-aged, don't look at me. On a tour of Bond Street and Oxford Street at the weekend, I was struck by the widow's-weeds look that has grown slowly, like a murky shadow, over every shop floor. One of the very best books about frocks is Dodie Smith's 1940s novel I Capture the Castle, in which the impoverished heroine's older sister makes a mercenary engagement and is sent from the country up to London to be dressed. There, to her surprise, she writes home, every chic woman wears nothing but a little black suit or a little black dress.

Black and tailoring are the attributes of the sophisticated urban woman, but they can make you feel as though the gloom of the Thatcher years has descended once again. If you wore a black bandage Lycra minidress and a slash of red lipstick when you were 24 in 1987, you are now staring down the barrel of a gun at 45. Second-time-round syndrome is unsettling; you look in the mirror and all you see is how much you have aged. The shops - Betty Jackson at the high end, COS and M&S on the high street - are showing some strong colours such as burnt orange and peacock blue, which, if your colouring allows you to carry them off, lift the spirits on a winter's day.

The other thing definitely on the list this autumn is a leather jacket. Every shop has got them. I don't mean those boring, ubiquitous "car coats", but a leather trench, a leather blazer or something a bit more biker/rock chick. Alexander McQueen has a leather jacket that is quilted like a Chanel handbag, and so, coming up, does M&S.

But if you really want to be "on trend", as they are always saying on the M&S website, you need a nice cardie, or "big knits" as they have been rebranded. What this means is not those little cashmere numbers, but a stonking great cable-knit thing that comes down to your knees over which you use a belt to accentuate your shape, should you have one. Some of the big knits, when you take them off the hanger, turn out to be cunningly disguised ponchos, which you'll put straight back again, because in the concise phrase of the Guardian's fashion editor, Jess Cartner-Morley, you don't want to look like a button mushroom.

I have so far neglected my apple-shaped friends, the ones with the undimpled knees and the ample bosoms. The idea, I believe, is that you make a waist with a belt. I've no idea if this works or not, as there is such a huge difference between my hips and my waist that I'm forever trying to even out the ratio, but I suspect that whatever gets moved in by the belt will simply be displaced upwards to the midriff. I'm trying to imagine how you are going to look in the other season's revival, jodhpurs, which I last remember seeing around 1990. The windows of Gap are full of them. The jodhpur, cut generously on the outside of the upper thigh, would, in principle, be a figure-fault fix for the fat-legged, but I can't get my head round the idea of making your thighs look bigger than they are already.

The other thing you need to get this season is a little black dress - assuming you don't have one already. Throughout the whole of her career, Chanel, its inventor in the 1920s, made LBDs in every fabric and to suit every shape. Her thesis was that when a woman walked into a room in an LBD, you didn't see the dress, you saw the woman. I can think of no better maxim when out shopping. For taste, knowing what to wear is about buying the right thing, not about buying for the sake of it.

This past summer with all its attendant miseries, its downpours, its gang murders, its stock exchange nightmares and its unwearable clothes, should have taught us to stand up to fashion. To buy in order to make us look good, not to be a perambulating advert for some scruffy graduate of Central St Martin's.